And pundits are talking about how hard it will be to defend "Obamacare" on the campaign trail later this year.

But the problem for Democrats in the Senate really comes down to one thing: They're defending a lot of seats this year in states that vote heavily for the Republicans.

Goldman Sachs is out with a big preview of the midterm elections, and though there's nothing too earth shattering in their forecasts, this chart nicely encapsulates the Democrats' problem.

On the left, you can see that there's only one competitive seat currently held by Republicans that's in a Democratic leaning state: That's in Maine, where Susan Collins is running for re-election. On the right, everywhere there's a blue arrow, there's a seat either held by a Democratic incumbent or being vacated by a Democrat that's in a Republican state. Plus there are a bunch of seats currently held by Democrats in states where the vote totals tend to be close.

So it's as simple as that: Senate Democrats are in trouble because this time around they hold a lot of seats in states with unfriendly terrain.